Estrogen use in general has been associated with increased risk of venous thromboemboli and has been shown to activate dose-dependently coagulation pathways. We have recently shown that estrogen therapy of conventional dosage potentiates fibrinolysis in postmenopausal women, a process that could be a consequence of primary estrogen-induced coagulation activation. To assess the mechanism of estrogen-induced fibrinolysis, we measured parameters of coagulation activation [prothrombin Framents1+2 (F1+2), thrombin-antithrombin (TAT)complexes] and parameters of fibrinolysis potentiation [tissue-type plasminogen activator (t-PA) and plasminogen activator inhibitor (PAI-1) activities] at baseline and following one month of conjugated equine estrogen .625 mg administered to 9 postmenopausal women. We found that conjugated equine estrogen marginally increased F1+2 by 18+/-31% (p=.14) and nonsignificantly increased TAT by 6+/-50% (p=1.00). Conjugated estrogen reduced PAI-1 activity by 38+/-30% (p=.03) and increased t-PA activity of 68+/-85% (p=.09). There was no association between % change in TAT and % change in t-PA (r=.05), and the associations between % change in TAT and % change in PAI-1 (R=.393), % change in F1+2 and % change in t-PA (r=-.60) or % change in F1+2 and % change in PAI-1 (r=.527) were directionally opposite anticipated correlations if potentiation of fibrinolysis was the result of coagulation activation. We conclude that potentiation of fibrinolysis is a primary effect of conjugated equine estrogen at the dosage used in our study and likely accounts for low risk of venous thromboemboli in postmenopausal women taking the conventional dosage of this preparation.